Alkali metal sulfur redox chemistry offers promising potential for high-energy-density energy storage. Fundamental understanding of alkali metal sulfur redox reactions is the prerequisite for rational designs of electrode and electrolyte. Here, we revealed a strong impact of alkali metal cation (Li, Na, K, and Rb) on polysulfide (PS) stability, redox reversibility, and solid product passivation. We employed operando UV-vis spectroscopy to show that strongly negatively charged short-chain PS (e.g., S/S) is more stabilized in the electrolyte with larger cation (e.g., Rb) than that with the smaller cation (e.g., Li), which is attributed to a stronger cation-anion electrostatic interaction between Rb and S/S owing to its weaker solvation energy. In contrast, Li is much more strongly solvated by solvent and thus exhibits a weaker electrostatic interaction with S/S. The stabilization of short-chain PS in K-, Rb-sulfur cells promotes the reduction of long-chain PS to short-chain PS, leading to high discharge potential. However, it discourages the oxidation of short-chain PS to long-chain PS, leading to poor charge reversibility. Our work directly probes alkali metal-sulfur redox chemistry in operando and provides critical insights into alkali metal sulfur reaction mechanism.

This work demonstrates an effective and universal strategy to improve the sluggish organosulfides (R–Sn–R) for redox flow batteries by asymmetric allylsubstituted organosulfides (R–Sn–A).

The lithium-sulfur (Li-S) battery is one of the promising energy storage alternatives because of its high theoretical capacity and energy density. Factors governing the stability of polysulfide intermediates in Li-S batteries are complex and are strongly affected by the solvent used. Herein, the polysulfide reduction and the bond cleavage reactions are calculated in different solvent environments by the density functional theory (DFT) methods. We investigate the relationship between the donor numbers (DN) as well as the dielectric constants (ε) of the solvent system and the relative stability of different polysulfide intermediates. Our results show that the polysulfide reduction mechanism is dominated by its tendency to form the ion-pair with Li+ in different organic solvents.